There are large individual differences among humans and animals in behavioral, physiological and toxicological responses to drugs of abuse. Many of these individual differences in behavioral responses to drugs display substantial genetic components. Transgenic animals provide means for approaching three interrelated goals: 1) Identification of gene elements that confer cell-type specific expression and may thus allow targeting of introduced genetic material to appropriate brain regions; 2) Elucidation of gene elements yielding transsynaptic gene regulation and thus allowing appropriate regulated expression of introduced genetic material; and 3) Ascertainment of biochemical and behavioral consequences of the introduction of or disruption of specific genes. Dopaminergic systems' involvement in central mechanisms of reward and reinforcement, and involvement of pre- and post-synaptic opioid peptide systems in the effects of opiate drugs has led to focus on these systems during this FY. Elements in the dopamine transporter gene's 5' flanking region that might confer its exquisite dopamine cell-specific expression were sought by cloning more than 20 kb of this sequence, and examining expression in different cultured cells and in transgenic animals. Tyrosine hydroxylase promoter, which can provide catecholamine-specific gene expression, was used to drive expression of dopamine transporter, interesting transporter mutants, and mu opiate receptor CDNAS. Preliminary analyses appear to reveal brain expression of MRNA but less expression of protein, while initial behavioral studies indicate a variable influence of transporter overexpression on exploratory, habituation, and cocaine-responsive behaviors.